/**
 * jquery.snow - jQuery Snow Effect Plugin
 *
 * Available under MIT licence
 *
 * @version 1 (21. Jan 2012)
 * @author Ivan Lazarevic
 * @requires jQuery
 * @see http://workshop.rs
 *
 * @params minSize - min size of snowflake, 10 by default
 * @params maxSize - max size of snowflake, 20 by default
 * @params newOn - frequency in ms of appearing of new snowflake, 500 by default
 * @params flakeColor - color of snowflake, #FFFFFF by default
 * @example $.fn.snow({ maxSize: 200, newOn: 1000 });
 */
(function($){
  $.fn.snow = function(options){
      var $flake     = $('<div id="flake" />').css({'position': 'absolute', 'top': '-50px'}).html('❄'),
        documentHeight = $(document).height(),
        documentWidth  = $(document).width(),
        defaults    = {
                  minSize   : 10,
                  maxSize   : 20,
                  newOn    : 500,
                  flakeColor : "#FFFFFF"
                },
        options     = $.extend({}, defaults, options);
      //setInterval-setInterval() 方法可按照指定的周期（以毫秒计）来调用函数或计算表达式。
      //开始一个周期-开始添加雪花
      var interval    = setInterval( function(){
        var startPositionLeft  = Math.random() * documentWidth - 100,
          startOpacity    = 0.5 + Math.random(),
          sizeFlake      = options.minSize + Math.random() * options.maxSize,
          endPositionTop   = documentHeight - 40,
          endPositionLeft   = startPositionLeft - 100 + Math.random() * 200,
          durationFall    = documentHeight * 10 + Math.random() * 5000;
        $flake
          .clone()
          .appendTo('body')
          .css(
            {
              left: startPositionLeft,
              opacity: startOpacity,
              'font-size': sizeFlake,
              color: options.flakeColor
            }
          )
          .animate(//增加雪花动态效果
            {
              top: endPositionTop,
              left: endPositionLeft,
              opacity: 0.2
            },
            durationFall,
            'linear',
            function() {
              $(this).remove()
            }
          );
      }, options.newOn);
      //结束周期-停止添加雪花
   
  };
})(jQuery);